1. Field of Invention
The present invention relates to a decoding method and an apparatus thereof for reading an optical disk, and more particularly, to an Eight to Fourteen Modulation (EFM) data decoding method for reading an optical disk.
2. Description of Related Art
In order to comply with configurations of optical disks and to increase reliability of reading data from an optical disk, a data is first modulated with EFM (eight-to-fourteen modulation) and Reed-Soloman coding, and then scrambled with an interleave method before storing thereto.
When burning a compact disk (CD), the status of a memory unit therein is not changed (i.e. burned) only if the corresponding data bit is logic 1. That is, the status of a memory unit is burned when a data bit is logic 1 and not burned when a data bit is logic 0. An 8-bit data is transform into a 14-bit data with the EFM method before storing to the disk. The 14-bit data modulated with EFM method complying with the following criteria, i.e., the data storing (burning) status is performed no shorter than 3 EFM clock cycles long, and no longer than 11 EFM clock cycles. Take a 14-bit data as an example, number of bits being logic 0 therein is no less than 2 nor larger than 10 for a data between at least a pair of two adjacent logic 1 bits.
The foregoing EFM modulation comply with a digital versatile disk (DVD) as well, whereas an 8-to-16-bit modulation is performed instead of conventional 8-to-14-bit. The burning criteria are thus modified that the burning status being no shorter than 3 EFM clock cycles, and being no longer than 11 EFM clock cycles.
As reading rate of an optical disk is increased, e.g. 40-time playback speed and up, scratches may be formed on the disk, as a result, the data bits may be unsuccessfully or erroneously read (e.g. logic 1 data being misread as logic 0). Also a 14-bit data not complying with EFM modulation criteria as described above or an erroneous 14-bit data complying with EFM modulation criteria may result in, for example, misread 00100010000100 as 00100001000100. When reading error occurs, an errant data is read from the optical disk and thus procedures thereafter are adversely influenced, e.g. error correction procedures being more burdened.